Drs. Brooks and Van Remmen have a strong record of collaboration in studies on the role of oxidative stress in sarcopenia. Our previous work demonstrated that young adult mice lacking the superoxide anion scavenger CuZnSOD (Sod1KO mice) exhibit age-related muscle atrophy/weakness that closely mimics the sarcopenia phenotype of old wild type (WT) mice, including degeneration of neuromuscular junctions (NMJ), retraction of motor neurons, elevated generation of muscle mitochondrial reactive oxygen species (mtROS), and altered calcium homeostasis. Replacing CuZnSOD specifically in neurons of Sod1KO mice reverses muscle atrophy and weakness, NMJ disruption and muscle oxidative stress, implicating motor neuron deficits as the initiating event in sarcopenia in Sod1KO mice. However, neuronal specific Sod1 knockout (nSod1KO) in mice does not result in atrophy of the gastrocnemius muscle, and although muscle specific Sod1 knockout mice show a loss in contractile force, they show no muscle atrophy. Thus, deletion of Sod1 and induction of oxidative stress in either neurons or muscle alone does not replicate the sarcopenia phenotype of the Sod1KO mice, suggesting that sarcopenia results from an interactive effect requiring both tissues. The goal of this study is to define this interaction. We hypothesize that pre-synaptic oxidative stress and damage initiates alterations in NMJ structure and function that trigger postsynaptic increases in mtROS generation, calcium dysregulation, and oxidative stress/damage in the muscle that further disrupt neuronal and NMJ function to generate the sarcopenia phenotype. We will address this hypothesis in three Specific Aims. First, we will examine the effect of Sod1 deficiencies in both neurons and muscle fibers with the expectation that these double knockout mice will recapitulate the phenotype of Sod1KO mice. We will also determine the effect of muscle or neuronal specific deficiency of Sod1 using conditional deletion of Sod1 during adulthood to determine the impact on sarcopenia, independent of possible developmental compensatory changes. Next, using additional conditional knockout models, we will test whether elevated muscle mtROS without presynaptic oxidative stress is sufficient to induce sarcopenia, and conversely, we will determine whether scavenging post- synaptic mtROS in Sod1KO will delay and/or reduce muscle atrophy and functional declines, despite neuronal changes present in the Sod1KO mice. In each model, we will measure oxidative damage and redox status in sciatic nerve, spinal cord, and muscle, NMJ morphology and function, motor unit properties, and skeletal muscle structure, mitochondrial function, calcium handling, and contractility. These studies will definitively show whether neuronal initiation of NMJ disruption is sufficient for muscle atrophy/weakness or if additional alterations in muscle oxidative stress are required to induce the phenotype. Identifying coordinated roles of neurons and muscle in the initiation and progression of sarcopenia will provide new insights into the pathways that are involved in the onset and propagation of sarcopenia.